The present invention relates to a rotor for electric motors The electric motors to which this invention relates are of the type where the rotors comprise permanent magnets, i.e. which do no include a slipring and which --in a particular case which will be described hereafter although the invention is by no means restricted to it --are similar in function to a synchronous machine and may be described also as brushless D.C. motors.
It is usual in the case of such electric motors to equip the rotor with permanent magnets distributed over the periphery of the disk in angle segments. The magnets used may in this case consist of flat-type magnets which may have a thickness of only 3 mm, but in any case a thickness somewhere in this range. These magnets are then operatively connected, electromagnetically, with magnetic fields resulting from coils arranged in axial layers with the rotor and acting on the magnets, for example, from both sides. The respective segment-type magnets of the rotor exhibit axial magnetization, it being desirable to keep the air gap to the adjoining structures generating the migrating magnetic fields as small as possible in the axial direction in order to achieve optimum efficiency.
However, problems have been encountered in such electric motors in connection with the mounting of the magnets constituting the rotor, which must be as precise as possible, to provide the smallest possible air gap and must, of course, be resistant to aging.
Consequently, it has been known to embed the partial magnets, which extend over given angular segments of a disk which then constitutes the rotor, in a plastic material so that a disk-shaped or wheel-shaped rotor is obtained. This method is, however, connected with the disadvantage that the plastic material may age or get distorted and that, in addition, much of the useful air gap is lost due to the fact that the magnets are covered (on both sides) by the plastic material.
According to another known method, the magnets are mounted on a hub fixed to the rotor shaft by means of metal disks (stainless steel disks) arranged on both sides. To this end, the magnet segments are inserted between the two disks and fixed in place, usually by gluing, in which case the arrangement is to be secured, however, by a retaining sleeve or bandage which has to be applied from the outside for securing the magnet segments, which have been glued to the metal disks on both sides, against the effects of the very high centrifugal forces. Such a bandage may, for example, consist of a peripheral composite glass fiber structure.
The overall structure of such a rotor comprising stainless steel disks on both sides and a peripheral composite glass fiber bandage is quite complex and therefore produces disadvantages as regards its structure and production. In addition, however, much of the useful air gap is lost in the case of such a structure due to the required sheet thickness and, in certain cases, also as a result of the cover layer over the magnets. Finally, the adhesive, which primarily secures the magnet segments in the two disks, is subjected mainly to shearing stresses --a type of stress which even high-quality adhesives are not always capable of withstanding.
Now, it is the object of the present invention to provide a rotor for an electric motor, where individual magnets (magnet segments) are arranged in the form of given angular segments over the periphery of the rotor, which on the one hand requires only little mechanical input and can, therefore, be produced at low cost and in a simple manner. On the other hand, the rotor retains the magnets in every respect safely and undetachably against the action of external forces, in particular against the action of centrifugal forces, without the need to apply a covering on the axial surfaces of the magnet segments.